1. Field of Industrial Utilization
The present invention relates to improvement of a member having an internal cooling passage, especially, to the improvement of a member having an internal cooling passage with a wall which possesses cooling ribs.
2. Description of the Prior Art
There are various members having an internal cooling passage, but the prior art is explained by a representative gas turbine blade as an example.
A gas turbine is an apparatus for converting high temperature and high pressure gas generated by the combustion of fuel with high pressure air compressed by a compressor as an oxidant to such an energy as electricity by driving a turbine.
Consequently, an increase in the electrical energy, that is obtained by consumption of a unit of fuel, is naturally preferable, and in view of the above described aspect, the improvement of the gas turbine performance is desired. And, as one of the methods for improvement of the gas turbine performed, the elevation of temperature and higher pressurizing of operating gas have been studied. On the other hand, a method for improvement of the total energy conversion efficiency of gas turbines and steam turbines by the elevation of operating gas temperature of the gas turbine and the combining with the steam turbine system utilizing high temperature exhaust gas in forming a combined plant has been proposed.
Operating gas temperature of the gas turbine is restricted by the durable capacity of the turbine blade material against hot corrosion resistance and thermal stress caused by the gas temperature. In elevating the operating gas temperature, a method for cooling the turbine blade by providing hollowed portions, namely a cooling flow passage, in the turbine blade itself, and flowing coolant such as air in the cooling flow passage is conventionally well adopted. More specifically, at least one cooling flow passage is formed inside of the turbine blade, for cooling the turbine blade from inside by flowing cooling air through the cooling flow passage, and, further, the surface, the top end, and the trailing edge of the turbine blade are cooled by releasing cooling air out of the blade through cooling holes provided at the above described cooling portions.
As for the above described cooling air, a part of air bled from a compressor is generally utilized. Accordingly, a large amount of cooling air consumption causes dilution of the gas the temperature and an increase of pressure loss. Therefore, it is important to cool effectively with a small quantity of cooling air.
For realizing a gas turbine having a higher gas operating temperature, it is important to improve heat transfer characteristics inside of the turbine blade for increased cooling effect of supplied cooling air, and various methods for heat transfer enhancement are used.
As one of the methods for heat transfer enhancement, there is a method of providing a plurality of ribs on the walls of cooling passages inside of the turbine blade because it is well known that the heat transfer coefficient can be improved by making an air flow on a thermal conducting plane surface turbulent or by breaking thermal boundary layers etc.
An example of the methods using a structure for heat transfer enhancement is disclosed in the reference, "Effects of Length and Configuration of Transverse Discrete Ribs on Heat Transfer and Friction for Turbulent Flow in a Square Channel", ASME/JSME Thermal Engineering Joint Conference, Vol. 3, pp. 213-218 (1991). The disclosed structure for heat transfer enhancement aims to improve heat transfer coefficient by arranging ribs having a length half of the width of the flow path at both the right and left sides of the flow path, alternately, the ribs extending in a direction perpendicular to the cooling air flow in order to break down the flow boundary layer and to increase turbulency of the cooling air flow with re-attaching flow. The ratio of the ribs pitch and the rib height is preferably about 10.
A second example of the methods using a structure for heat transfer enhancement is disclosed in the reference, "Heat Transfer Enhancement in Channels with Turbulence Promoters", ASME/84-WT/H-72 (1984). The disclosed structure for heat transfer enhancement aims to improve the transfer coefficient by using ribs arranged perpendicularly or slantingly to the cooling air flow in order to obtain the same effect as the above described first example. The slanting angle of the rib to the air flow is preferably from 60.degree. to 70.degree.. And, the ratio of the ribs pitch and the rib height is preferably about 10. An example utilizing the above described structure of the second example and which is further improved in heat transfer coefficient is disclosed in JP-A-60-101202 (1985). The disclosed structure for heat transfer enhancement in this reference is a structure having ribs arranged slantingly to the cooling air flow and additionally having machined slits therein. With the such a rib structure for heat transfer enhancement, it is said that further high cooling performance is realized by the turbulence of air flow behind the slit, and the slit hinders the accumulation of dust around the ribs and, consequently, prevents the lowering of heat transfer coefficient.
As the extracted air sent by a compressor is used for cooling of the turbine blade as previously described, there is an increase of cooling air consumption which lowers the thermal efficiency of the gas turbine. Accordingly, it is important to cool the gas turbine effectively with a small amount of cooling air. But, the above described conventional cooling structure of the turbine blade needs more cooling air in order to meet the elevating of the operation gas temperature of the turbine to a higher temperature, and the improvement of thermal efficiency of the gas turbine is generally small.